System and method for liquefying natural gas

ABSTRACT

Provided is a system for liquefying natural gas including: a pre-cooling means; a gas-liquid separating means; a first heat exchanging means; a second heat exchanging means; a first expanding means; a second expanding means; a first mixed refrigerant converting means; a pre-cooled refrigerant supplying means; a natural gas supplying means; and a mixer. Provided is also a method for liquefying natural gas including: a first pre-cooling operation (S 01 ); a first mixed refrigerant separating operation (S 02 ); a first introducing operation (S 03 ); a first expanded refrigerant forming operation (S 04 ); a second introducing operation (S 05 ); a second expanded refrigerant forming operation (S 06 ); and a second cooling operation (S 07 ).

TECHNICAL FIELD

The present invention relates to a system and a method for liquefying natural gas, and more particularly, to a system and a method for liquefying natural gas using freezing, liquefaction or coagulation.

BACKGROUND ART

A thermodynamic process of liquefying natural gas to produce liquefied natural gas (LNG) has been developed since the 1970s in order to satisfy various problems including demands for higher efficiency and larger capacity. In order to satisfy these demands, that is, increase efficiency and capacity of a liquefying process, various attempts to liquefy natural gas using different refrigerants or different cycles have been continuously conducted up to now. However, currently, the number of liquefaction processes that are practically used is very small.

One of the liquefaction processes that are being operated and have been most widely spread is a ‘propane pre-cooled mixed refrigerant process (or a C3/MR process)’.

FIG. 1 is a flowchart of a C3/MR process.

As shown in FIG. 1, in the C3/MR process, natural gas is pre-cooled up to approximately 238 K by a multi-stage of propane (C3) Joule-Thomson (JT) cycle. The pre-cooled natural gas is liquefied and sub-cooled up to 123 K by heat exchange with a mixed refrigerant (MR) in a heat exchanger.

In this case, in describing a freezing cycle of the mixed refrigerant in more detail, the mixed refrigerant is compressed at high pressure, cooled, and then introduced into a gas-liquid separator 10.

The mixed refrigerant is separated into a gas state (light components) and a liquid state (heavy components) by the gas-liquid separator 10, which are each introduced into a primary heat exchanger 20, wherein the mixed refrigerant in the liquid state is expanded once a primary heat exchange is completed in the primary heat exchanger 20, and is used for cooling a high temperature stream introduced into the primary heat exchanger 20. The mixed refrigerant in the gas state is introduced into a secondary heat exchanger 30 and is cooled therein. The mixed refrigerant in the gas state is further cooled by expansion and is used for cooling the secondary heat exchanger 30 and the primary heat exchanger 20.

The above-mentioned C3/MR process has a disadvantage that the heat exchangers 20 and 30 have low heat exchanging efficiency.

As the related art, U.S. Pat. No. 6,691,531 B1 has been suggested and various developments of a system for liquefying natural gas for solving the above-mentioned problem have been required.

RELATED ART DOCUMENT

(Patent Document 1) U.S. Pat. No. 6,691,531 B1 (2004.02.17)

DISCLOSURE Technical Problem

An object of the present invention is to provide a system and a method for liquefying natural gas capable of maximizing heat exchanging efficiency and energy saving efficiency by improving a configuration of a heat exchanger.

Technical Solution

In one general aspect, a system for liquefying natural gas includes: a pre-cooling means 100; a gas-liquid separating means 200 connected to the pre-cooling means 100; a first heat exchanging means 300 connected to the pre-cooling means 100 and the gas-liquid separating means 200, respectively; a second heat exchanging means 400 connected to the first heat exchanging means 300; a first expanding means 510 having one end which is connected to the first heat exchanging means 300; a second expanding means 520 having both ends which are each connected to the second heat exchanging means 400; a first mixed refrigerant converting means 600 connected to the pre-cooling means 100 and the first heat exchanging means 300, respectively; a pre-cooled refrigerant supplying means 700 connected to the pre-cooling means 100; a natural gas supplying means 800 connected to the pre-cooling means 100; and a mixing means 900 connecting the other end of the first expanding means 510 and the first heat exchanging means 300 to each other and connected to the second heat exchanging means 400.

The first heat exchanging means 300 may be a first heat exchanger 310 connected to the pre-cooling means 100 and the gas-liquid separating means 200, and the second heat exchanging means 400 may be a third heat exchanger 410 connected to the first heat exchanger 310.

The first heat exchanging means 300 may be a first heat exchanger 310 connected to the pre-cooling means 100 and the gas-liquid separating means 200 and a second heat exchanger 320 connected to the first heat exchanger 310, and the second heat exchanging means 400 may be a third heat exchanger 410 connected to the second heat exchanger 320 and a fourth heat exchanger 420 connected to the third heat exchanger 410.

The first expanding means 510 may have one end connected to the first heat exchanger 310 and the other end connected to the second heat exchanger 320, and the second expanding means 520 may have one end connected to the third heat exchanger 410 and the other end connected to the fourth heat exchanger 420.

The first mixed refrigerant converting means 600 may supply a first mixed refrigerant to the pre-cooling means 100, the pre-cooled refrigerant supplying means 700 may supply a pre-cooled refrigerant to the pre-cooling means 100, the natural gas supplying means 800 may supply natural gas to the pre-cooling means 100, the pre-cooling means 100 may pre-cool the first mixed refrigerant and the natural gas which are each supplied from the pre-cooling means 100 and the natural gas supplying means 800 using the pre-cooled refrigerant supplied from the first mixed refrigerant converting means 600, the gas-liquid separating means 200 may separate the first mixed refrigerant introduced from the pre-cooling means 100 into a first separated refrigerant of a liquid state and a second separated refrigerant of a gas state, the first heat exchanging means 300 may introduce the natural gas, the first separated refrigerant, and the second separated refrigerant from the pre-cooling means 100, cool the natural gas, the first separated refrigerant, and the second refrigerant using a mixed refrigerant, and form the first separated refrigerant at high temperature by cooling less the first separated refrigerant than the natural gas and the second separated refrigerant, the first expanding means 510 may form a first expanded refrigerant by expanding the first separated refrigerant introduced from the first heat exchanging means 300, the second heat exchanging means 400 may introduce the natural gas and the second separated refrigerant from the first heat exchanging means 300, cool the natural gas and the second separated refrigerant using a second expanded refrigerant, form the second separated refrigerant at high temperature by cooling less the second separating refrigerant than the natural gas, and form liquefied natural gas by over-cooling the natural gas, the second expanding means 520 may supply the second expanded refrigerant formed by expanding the second separated refrigerant introduced from the second heat exchanging means 400 to the second heat exchanging means 400, and the mixing means 900 may supply the mixed refrigerant formed by mixing a portion of the first expanded refrigerant introduced from the first expanding means 510 and the second expanded refrigerant introduced from the second heat exchanging means 400 to the first heat exchanging means 300.

The pre-cooled refrigerant may be a single refrigerant or a second mixed refrigerant.

The first mixed refrigerant converting means 600 may convert a mixed refrigerant introduced from the first heat exchanging means 300 into a first mixed refrigerant by sequentially compressing and cooling the mixed refrigerant introduced from the first heat exchanging means 300 and supply the first mixed refrigerant to the pre-cooling means 100.

In another general aspect, a method for liquefying natural gas includes: a first pre-cooling operation S01 pre-cooling a first mixed refrigerant and natural gas; a first mixed refrigerant separating operation S02 separating the first mixed refrigerant into a first separated refrigerant of a liquid state and a second separated refrigerant of a gas state, respectively; a first introducing operation S03 introducing the natural gas, the first separated refrigerant, and the second separated refrigerant into a first heat exchanging region without being mixed; a first expanded refrigerant forming operation S04 forming a first expanded refrigerant by introducing the first separated refrigerant into a first expanding region and expanding the first separated refrigerant which is introduced into the first expanding region; a second introducing operation S05 introducing the natural gas and the second separated refrigerant into a second heat exchanging region without being mixed; a second expanded refrigerant forming operation S06 forming a second expanded refrigerant by introducing the second separated refrigerant into a second expanding region and expanding the second separated refrigerant which is introduced into the second expanding region; an over-cooling operation S07 cooling the natural gas and the second separated refrigerant which were introduced into the second heat exchanging region by supplying the second expanded refrigerant to the second heat exchanging region, forming the second separated refrigerant at high temperature by cooling less the second separated refrigerant than the natural gas, and forming liquefied natural gas by over-cooling the natural gas; a mixed refrigerant forming operation S08 forming a mixed refrigerant by mixing the second expanded refrigerant and the first expanded refrigerant; and a cooling operation S09 cooling the natural gas, the first separated refrigerant, and the second separated refrigerant which were introduced into the first heat exchanging region by supplying the mixed refrigerant to the first heat exchanging region, and forming the first separated refrigerant at high temperature by cooling less the first separated refrigerant than the natural gas and the second separated refrigerant.

In the first pre-cooling operation S01, the first mixed refrigerant and the natural gas may be pre-cooled using a single refrigerant or a second mixed refrigerant.

The method for liquefying natural gas may further include: a converting operation S10 converting the mixed refrigerant into the first mixed refrigerant by sequentially compressing and cooling the mixed refrigerant; a second pre-cooling operation S11 pre-cooling the first mixed refrigerant and the natural gas; and a repetition cycle operation S12 repeating the first mixed refrigerant separating operation S02 to the second pre-cooling operation S11 as one period one or more times.

In the second pre-cooling operation S11, the first mixed refrigerant and the natural gas may be pre-cooled using a single refrigerant or a second mixed refrigerant.

Advantageous Effects

According to the present invention, the temperature difference between the refrigerant introduced into the first heat exchanging means and the natural gas is decreased, such that the energy consumption for liquefying the natural gas may be saved.

DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart of a conventional C3/MR process.

FIG. 2 is a system for liquefying natural gas according to a first exemplary embodiment of the present invention.

FIG. 3 is a system for liquefying natural gas according to a second exemplary embodiment of the present invention.

FIG. 4 is a system for liquefying natural gas according to a third exemplary embodiment of the present invention.

FIG. 5 is a system for liquefying natural gas according to a fourth exemplary embodiment of the present invention.

FIG. 6 is a method for liquefying natural gas according to the present invention.

BEST MODE

Hereinafter, a technical spirit of the present invention will be described in more detail with reference to the accompanying drawings.

The accompanying drawings are only examples shown in order to describe the technical spirit of the present invention in more detail. Therefore, the technical spirit of the present invention is not limited to shapes of the accompanying drawings.

FIG. 2 is a system for liquefying natural gas according to a first exemplary embodiment of the present invention.

As shown in FIG. 2, a system 1000 a for liquefying natural gas according to a first exemplary embodiment of the present invention is configured to include a pre-cooling means 100, a gas-liquid separating means 200, a first heat exchanging means 300, a second heat exchanging means 400, a first expanding means 510, a second expanding means 520, a first mixed refrigerant converting means 600, a pre-cooled refrigerant supplying means 700, a natural gas supplying means 800, and a mixing means 900.

The components of the system 1000 a for liquefying natural gas according to the first exemplary embodiment of the present invention are connected to each other by a plurality of pipes. A detailed description thereof is as follows.

A first pipe sequentially connects the natural gas supplying means 800, the pre-cooling means 100, the first heat exchanging means 300, and the second heat exchanging means 400, and is connected to the pre-cooling means 100, the first heating exchanging means 300, and the second heat exchanging means 400, respectively, so as to penetrate therethrough.

A second pipe, which has a cycle shape, connects the pre-cooled refrigerant supplying means 700 and the pre-cooling means 100 to each other.

A third pipe sequentially connects the first mixed refrigerant converting means 600, the pre-cooling means 100, and the gas-liquid separating means 200, and is connected to the pre-cooling means 100 so as to penetrate therethrough.

A fourth pipe sequentially connects the gas-liquid separating means 200, the first heat exchanging means 300, the first expanding means 510, the mixing means 900, the first heat exchanging means 300, and the first mixed refrigerant converting means 600, and is connected to the first heat exchanging means 300 so as to penetrate therethrough two times.

Particularly, in connecting the gas-liquid separating means 200, the first heat exchanging means 300, and the first expanding means by the fourth pipe, the fourth pipe is connected from the gas-liquid separating means 200 to a central portion of the first heat exchanging means 300, is led to a lower side or an upper side of the first heat exchanging means 300, and is then connected to the first expanding means 510.

That is, the fourth pipe is formed so as not to straightly penetrate through the first heat exchanging means 300.

A fifth pipe sequentially connects the gas-liquid separating means 200, the first heat exchanging means 300, the second heat exchanging means 400, the second expanding means 520, the second heat exchanging means 400, and the mixing means 900, is connected to the first heat exchanging means 300 so as to penetrate therethrough, and is connected to the second heat exchanging means 400 so as to penetrate therethrough two times.

Next, the components of the system for liquefying natural gas according to the first exemplary embodiment of the present invention will be described in detail.

The first mixed refrigerant converting means 600 is configured to include a first MR compressor 610 and a first MR cooler 620.

The first MR compressor 610 is configured by a three-stage compressor and compresses the first mixed refrigerant.

The first MR cooler 620 introduces the first mixed refrigerant which is compressed in the first MR compressor 610 thereinto, cools the first mixed refrigerant and then supplies it to the pre-cooling means 100.

That is, the first mixed refrigerant converting means 600 has a configuration for compressing and cooling the first mixed refrigerant and supplying it to the pre-cooling means 100.

The pre-cooled refrigerant supplying means 700 is configured to include a C3 compressor 710, a C3 cooler 720, and a C3 expansion valve 730.

In addition, the pre-cooled refrigerant supplying means 700 sequentially compresses, cools, and expands a single refrigerant using the C3 compressor 710, the C3 cooler 720, and the C3 expansion valve 730, and supplies it to the pre-cooling means 100.

In this case, as the single refrigerant, propane is used, the C3 compressor 710 is configured by a four-stage compressor, and the C3 expansion valve 730 is configured by a four-stage Joule-Thomson valve.

The natural gas supplying means 800, which is a tank in which natural gas is stored, supplies the stored natural gas to the pre-cooling means 100.

The pre-cooling means 100 independently introduces the first mixed refrigerant from the first mixed refrigerant converting means 600, the single refrigerant from the pre-cooled refrigerant supplying means 700, and the natural gas from the natural gas supplying means 800, respectively.

In addition, the pre-cooling means 100 pre-cools the first mixed refrigerant and the natural gas using the single refrigerant.

The gas-liquid separating means 200 introduces the first mixed refrigerant from the pre-cooling means 100 and separates it into a first separated refrigerant in a liquid state and a second separated refrigerant in a gas state, respectively.

The first heat exchanging means 300 is configured by a first heat exchanger 310, independently introduces the natural gas from the pre-cooling means 100, independently introduces the first separated refrigerant and the second separated refrigerant, respectively, from the gas-liquid separating means 200, cools the natural gas, the first separated refrigerant and the second separated refrigerant using a mixed refrigerant introduced into the first heat exchanging means 300 through several post-processes, and forms the first separated refrigerant at high temperature by cooling less the first separated refrigerant than the natural gas and the second separated refrigerant.

Here, the mixed refrigerant will be described below.

The first expanding means 510, which is an expansion valve disposed at the lower side or the upper side of the first heat exchanger 310, is connected to the central portion of the first heat exchanger 310 by the fourth pipe to forcedly introduce and expand the first separated refrigerant from the first heat exchanger 310, thereby forming a first expanded refrigerant.

That is, the first expanding means 510 forcedly extracts the first separated refrigerant introduced into the first heat exchanger 310 from the central portion of the first exchanger 310.

The second heat exchanging means 400 is configured by a third heat exchanger 410, independently introduces the second separated refrigerant and the natural gas from the first heat exchanger 310, and cools the natural gas and the second separated refrigerant using a second expanded refrigerant introduced into the second heat exchanging means 400 through several post-processes, wherein the second heat exchanging means 400 forms the second separated refrigerant at high temperature by cooling less the second separated refrigerant than the natural gas and forms liquefied natural gas by over-cooling the natural gas.

Here, the second expanded refrigerant will be described below.

The second expanding means 520, which is an expansion valve, forms the second expanded refrigerant by introducing and expanding the second separated refrigerant from the third heat exchanger 410 and supplies the second expanded refrigerant which is formed to the third heat exchanger 410.

The mixing means 900 supplies the mixed refrigerant formed by mixing the first expanded refrigerant introduced from the first expanding means 510 and the second expanded refrigerant introduced from the third heat exchanger 410 to the first heat exchanger 310.

Consequently, according to the present invention, as a final discharging temperature difference between the mixed refrigerant introduced from the mixing means 900 to the first heat exchanging means 300 and the natural gas and the second separated refrigerant which are cooled in the first heat exchanging means 300 is decreased, heat exchanging efficiency is increased, thereby making it possible to reduce energy consumption for liquefying the natural gas.

Meanwhile, the first mixed refrigerant converting means 600 sequentially compresses and cools the mixed refrigerant introduced from the first heat exchanger 310 to again supply it to the pre-cooling means 100.

That is, the first mixed refrigerant converting means 600 converts the mixed refrigerant into the first mixed refrigerant and again supplies it to the pre-cooling means 100.

Experiment results of a process for liquefying natural gas using a system 1000 a for liquefying natural gas according to the first exemplary embodiment of the present invention are as follows.

TABLE 1 Component Mol % Nitrogen 0.22 Methane 91.33 Ethane 5.36 Propane 2.14 I-butane 0.46 n-butane 0.47 I-pentane 0.01 n-pentane 0.01

TABLE 2 Pressure(Bar) 53 Temperature(° C.) 45 Flow Rate(kmol/hr) 35065

The natural gas is composed of compositions illustrated in Table 1, has pressure and temperature illustrated in Table 2, and sequentially passes through the pre-cooling means 100, the first heat exchanging means 300, and the second heat exchanging means 400.

The single refrigerant (propane) is compressed in four stages by the C3 compressor 710 of the pre-cooled refrigerant supplying means 700 to have pressure of 16.4 bar, is expanded in a four stages by the C3 expander 730 to sequentially have pressure of 7.5 bar, 4.2 bar, 2.5 bar, 1.114 bar, and is supplied to the pre-cooling means 100.

TABLE 3 mole fraction (—) N2 0.0827 C1 0.4555 C2 0.3062 C3 0.1555

The first mixed refrigerant is composed of compositions illustrated in Table 3, is compressed in three-stages by the first MR compressor 610 of the first mixed refrigerant converting means 600 to have pressure of 60 bar, is pre-cooled through the pre-cooling means 100, and is then separated into the first separated refrigerant of the liquid state and the second separated refrigerant of the liquid state through the gas-liquid separator 200.

The first separated refrigerant is introduced into the first expanding means 510 through the first heat exchanging means 300 and is expanded therein, is formed to be the first expanded refrigerant having pressure of 4 bar, and then introduced into the mixer 900.

The second separated refrigerant is introduced into the second expanding means 520 through the second heat exchanging means 400 and expanded therein, next formed to be the second expanded refrigerant, and then introduced into the mixer 900 through the second heat exchanging means 400 again.

The mixer 900 introduces the mixed refrigerant formed by mixing the first expanded refrigerant and the second expanded refrigerant into the first heat exchanging means 300.

Meanwhile, the first separated refrigerant, the second separated refrigerant, and the natural gas which are introduced into the first heat exchanging means 300 are cooled by the mixed refrigerant introduced from the mixer 900.

In this case, a temperature difference between the first separated refrigerant, the second separated refrigerant, and the natural gas which are introduced into the first heat exchanging means 300 and the mixed refrigerant which is again introduced into the first heat exchanging means 300 through several processes was maintained at 4K, and power consumed in liquefying the natural gas in the case the above-mentioned temperature difference is maintained was 203900 KW.

Power consumed for liquefying the natural gas using a general C3/MR process is 210700 KW. Therefore, the system for liquefying natural gas according to the first exemplary embodiment of the present invention may reduce the power as much as 6800 KW as compared to the general C3/MR process.

The present applicant has empirically and experimentally devised the experiment results as described above.

FIG. 3 is a system for liquefying natural gas according to a second exemplary embodiment of the present invention.

As shown in FIG. 3, a system 1000 b for liquefying natural gas according to the second exemplary embodiment of the present invention has the same configuration as that of the system 1000 a for liquefying natural gas according to the first exemplary embodiment of the present invention, but has different configurations of the first pipe, the fourth pipe, the fifth pipe, the first heat exchanging means 300, and the second heat exchanging means 400.

The first heat exchanging means 300 is configured to include a first heat exchanger 310 and a second heat exchanger 320, and the second exchanging means 400 is configured to include a third heat exchanger 410 and a fourth heat exchanger 420.

A first pipe sequentially connects the natural gas supplying means 800, the pre-cooling means 100, the first heat exchanger 310, the second heat exchanger 320, the third heat exchanger 410, and the fourth heat exchanger 420, and is connected to the pre-cooling means 100, the first heat exchanger 310, the second heat exchanger 320, the third heat exchanger 410, and the fourth heat exchanger 420, respectively, so as to penetrate therethrough.

A fourth pipe sequentially connects the gas-liquid separating means 200, the first heat exchanger 310, the first expanding means 510, the mixing means 900, the second heat exchanger 320, the first heat exchanger 310, and the first mixed refrigerant converting means 600, and is connected to the second heat exchanger 320 so as to penetrate therethrough, and connected to the first heat exchanger 310 so as to penetrate therethrough two times.

A fifth pipe sequentially connects the gas-liquid separating means 200, the first heat exchanger 310, the second heat exchanger 320, the third heat exchanger 410, the second expanding means 520, the fourth heat exchanger 420, the third heat exchanger 410, and the mixing means 900, is connected to the first heat exchanger 310, the second heat exchanger 320, and the fourth heat exchanger 420 so as to penetrate therethrough, and connected to the third heat exchanger 410 so as to penetrate therethrough two times.

According to the system 1000 b for liquefying natural gas according to the second exemplary embodiment of the present invention, the first heat exchanging means 300 includes the first heat exchanger 310 and the second heat exchanger 320, such that the system 1000 b for liquefying natural gas according to the second exemplary embodiment of the present invention may have the same effect as that of the configuration of the first heat exchanging means 300 according to the first exemplary embodiment of the present invention.

In addition, according to the system 1000 b for liquefying natural gas according to the second exemplary embodiment of the present invention, the second heat exchanging means 400 includes the third heat exchanger 410 and the fourth heat exchanger 420, such that the system 1000 b for liquefying natural gas according to the second exemplary embodiment of the present invention may have the same effect as that of the configuration of the second heat exchanging means 400 according to the first exemplary embodiment of the present invention.

FIG. 4 is a system for liquefying natural gas according to a third exemplary embodiment of the present invention.

As shown in FIG. 4, a system 1000 c for liquefying natural gas according to the third exemplary embodiment of the present invention has the same configuration as that of the system 1000 a for liquefying natural gas according to the first exemplary embodiment of the present invention, but has a different configuration of the pre-cooled refrigerant supplying means 700.

The pre-cooled refrigerant supplying means 700, which is a configuration for providing the second mixed refrigerant to the pre-cooling means 100, is configured to include a second MR compressor 740, a second MR cooler 750, and a second MR expansion valve 760.

That is, the pre-cooled refrigerant supplying means 700 supplies the second mixed refrigerant to the pre-cooling means 100 using the second MR compressor 740, the second MR cooler 750, and the second MR expansion valve 760.

Here, the second mixed refrigerant is formed of the same material as that of the first mixed refrigerant.

FIG. 5 is a system for liquefying natural gas according to a fourth exemplary embodiment of the present invention.

As shown in FIG. 5, a system 1000 d for liquefying natural gas according to the fourth exemplary embodiment of the present invention has the same configuration as that of the system 1000 b for liquefying natural gas according to the second exemplary embodiment of the present invention, and has the same configuration of the pre-cooling refrigerant supplying means as that of the third exemplary embodiment of the present invention.

FIG. 6 is a method for liquefying natural gas according to the present invention.

As shown in FIG. 6, a method for liquefying natural gas according to the present invention is configured to include a pre-cooling operation (S01), a first mixed refrigerant separating operation (S02), a first introducing operation (S03), a first expanded refrigerant forming operation (S04), a second introducing operation (S05), a second expanded refrigerant forming operation (S06), an over-cooling operation (S07), a mixed refrigerant forming operation (S08), a cooling operation (S09), a converting operation (S10), a second pre-cooling operation (S11), and a repetition cycle operation (S12).

The method for liquefying natural gas according to the present invention will be described in detail with reference to FIG. 6.

First, the first mixed refrigerant and the natural gas which are supplied from the outside are pre-cooled using the single refrigerant or the second mixed refrigerant. This corresponds to the first pre-cooling operation (S01) shown in FIG. 6.

Next, the first mixed refrigerant which is pre-cooled is separated into the first separated refrigerant of the liquid state and the second separated refrigerant of the gas state, respectively. This corresponds to the first mixed refrigerant separating operation (S02) shown in FIG. 6.

Next, the natural gas, the first separated refrigerant, and the second refrigerant which are pre-cooled are introduced into a first heat exchanging region without being mixed. This corresponds to the first introducing operation (S03) shown in FIG. 6. Meanwhile, the natural gas, the first separated refrigerant, and the second separated refrigerant which are introduced into the first heat exchanging region are cooled by the mixed refrigerant, which will be described below.

Next, the first separated refrigerant is introduced into the first expanded region and is expanded therein, thereby forming the first expanded refrigerant. This corresponds to the first expanded refrigerant forming operation (S04) shown in FIG. 6.

Next, the natural gas and the second separated refrigerant are introduced into the second heat exchanging region without being mixed. This corresponds to the second introducing operation (S05) shown in FIG. 6.

Next, the second separated refrigerant is introduced into the second expanded region and is expanded therein, thereby forming the second expanded refrigerant. This corresponds to the second expanded refrigerant forming operation (S06) shown in FIG. 6.

Next, the second expanded refrigerant is supplied to the second heat exchanging region to cool the natural gas and the second separated refrigerant which were introduced into the second heat exchanging region in the second introducing operation (S05), wherein the second separated refrigerant is formed at high temperature by cooling less the second separated refrigerant than the natural gas and the liquefied natural gas is formed by over-cooling the natural gas. This corresponds to the over-cooling operation (S07) shown in FIG. 6.

Next, the mixed refrigerant is formed by mixing the second expanded refrigerant used in the over-cooling operation (S07) and the first expanded refrigerant formed in the first expanded refrigerant forming operation (S04). This corresponds to the mixed refrigerant forming operation (S08) shown in FIG. 6.

Next, the mixed refrigerant is supplied to the first heat exchanging region to cool the natural gas, the first separated refrigerant, and the second separated refrigerant which were introduced into the first heat exchanging region in the first introducing operation (S03), wherein the first separated refrigerant is formed at high temperature by cooling less the first separated refrigerant than the natural gas and the second separated refrigerant. This corresponds to the cooling operation (S09) shown in FIG. 6.

Next, the mixed refrigerant which was introduced into the first heat exchanging region is introduced into a converting region, and is compressed and cooled therein, thereby converting it into the first mixed refrigerant. This corresponds to the converting operation (S10) shown in FIG. 6.

Next, the first mixed refrigerant generated in the converting operation (S10) and the natural gas which is supplied from the outside are pre-cooled using the single refrigerant or the second mixed refrigerant. This corresponds to the second pre-cooling operation (S11) shown in FIG. 6.

Next, the first mixed refrigerant separating operation S02 to the second pre-cooling operation S11 are repeated as one period one or more times. This corresponds to the repetition cycle operation (S12) shown in FIG. 6.

The present invention is not limited to the above-mentioned exemplary embodiments, and may be variously applied, and may be variously modified without departing from the gist of the present invention claimed in the claims.

DETAILED DESCRIPTION OF MAIN ELEMENTS

-   -   1000 a, b, c, d: system for liquefying natural gas according to         the present invention     -   100: pre-cooling means     -   200: gas-liquid separating means     -   300: first heat exchanging means     -   310: first heat exchanger 320 second heat exchanger     -   400: second heat exchanging means     -   410: third heat exchanger 420 fourth heat exchanger     -   510: first expanding means     -   520: second expanding means     -   600: first mixed refrigerant converting means     -   700: pre-cooled refrigerant supplying means     -   800: natural gas supplying means     -   900: mixing means 

1. A system for liquefying natural gas, comprising: a pre-cooling means; a gas-liquid separating means connected to the pre-cooling means; a first heat exchanging means connected to the pre-cooling means and the gas-liquid separating means, respectively; a second heat exchanging means connected to the first heat exchanging means; a first expanding means having one end which is connected to the first heat exchanging means; a second expanding means having both ends which are each connected to the second heat exchanging means; a first mixed refrigerant converting means connected to the pre-cooling means and the first heat exchanging means, respectively; a pre-cooled refrigerant supplying means connected to the pre-cooling means; a natural gas supplying means connected to the pre-cooling means; and a mixing means connecting the other end of the first expanding means and the first heat exchanging means to each other and connected to the second heat exchanging means.
 2. The system for liquefying natural gas of claim 1, wherein the first heat exchanging means is a first heat exchanger connected to the pre-cooling means and the gas-liquid separating means, and the second heat exchanging means is a third heat exchanger connected to the first heat exchanger.
 3. The system for liquefying natural gas of claim 1, wherein the first heat exchanging means is a first heat exchanger connected to the pre-cooling means and the gas-liquid separating means, and a second heat exchanger connected to the first heat exchanger, and the second heat exchanging means is a third heat exchanger connected to the second heat exchanger, and a fourth heat exchanger connected to the third heat exchanger.
 4. The system for liquefying natural gas of claim 3, wherein the first expanding means has one end connected to the first heat exchanger and the other end connected to the second heat exchanger, and the second expanding means has one end connected to the third heat exchanger and the other end connected to the fourth heat exchanger.
 5. The system for liquefying natural gas of claim 1, wherein the first mixed refrigerant converting means supplies a first mixed refrigerant to the pre-cooling means, the pre-cooled refrigerant supplying means supplies a pre-cooled refrigerant to the pre-cooling means, the natural gas supplying means supplies natural gas to the pre-cooling means, the pre-cooling means pre-cools the first mixed refrigerant and the natural gas which are each supplied from the pre-cooling means and the natural gas supplying means using the pre-cooled refrigerant supplied from the first mixed refrigerant converting means, the gas-liquid separating means separates the first mixed refrigerant introduced from the pre-cooling means into a first separated refrigerant of a liquid state and a second separated refrigerant of a gas state, the first heat exchanging means introduces the natural gas, the first separated refrigerant, and the second separated refrigerant from the pre-cooling means, cools the natural gas, the first separated refrigerant, and the second refrigerant using a mixed refrigerant, and forms the first separated refrigerant at high temperature by cooling less the first separated refrigerant than the natural gas and the second separated refrigerant, the first expanding means forms a first expanded refrigerant by expanding the first separated refrigerant introduced from the first heat exchanging means, the second heat exchanging means introduces the natural gas and the second separated refrigerant from the first heat exchanging means, cools the natural gas and the second separated refrigerant using a second expanded refrigerant, forms the second separated refrigerant at high temperature by cooling less the second separating refrigerant than the natural gas, and forms liquefied natural gas by over-cooling the natural gas, the second expanding means supplies the second expanded refrigerant formed by expanding the second separated refrigerant introduced from the second heat exchanging means to the second heat exchanging means, and the mixing means supplies the mixed refrigerant formed by mixing a portion of the first expanded refrigerant introduced from the first expanding means and the second expanded refrigerant introduced from the second heat exchanging means to the first heat exchanging means.
 6. The system for liquefying natural gas of claim 5, wherein the pre-cooled refrigerant is a single refrigerant or a second mixed refrigerant.
 7. The system for liquefying natural gas of claim 5, wherein the first mixed refrigerant converting means converts a mixed refrigerant introduced from the first heat exchanging means into a first mixed refrigerant by sequentially compressing and cooling the mixed refrigerant introduced from the first heat exchanging means and supplies the first mixed refrigerant to the pre-cooling means.
 8. A method for liquefying natural gas, comprising: a first pre-cooling operation pre-cooling a first mixed refrigerant and natural gas; a first mixed refrigerant separating operation separating the first mixed refrigerant into a first separated refrigerant of a liquid state and a second separated refrigerant of a gas state, respectively; a first introducing operation introducing the natural gas, the first separated refrigerant, and the second separated refrigerant into a first heat exchanging region without being mixed; a first expanded refrigerant forming operation forming a first expanded refrigerant by introducing the first separated refrigerant into a first expanding region and expanding the first separated refrigerant which is introduced into the first expanding region; a second introducing operation introducing the natural gas and the second separated refrigerant into a second heat exchanging region without being mixed; a second expanded refrigerant forming operation forming a second expanded refrigerant by introducing the second separated refrigerant into a second expanding region and expanding the second separated refrigerant which is introduced into the second expanding region; an over-cooling operation cooling the natural gas and the second separated refrigerant which were introduced into the second heat exchanging region by supplying the second expanded refrigerant to the second heat exchanging region, forming the second separated refrigerant at high temperature by cooling less the second separated refrigerant than the natural gas, and forming liquefied natural gas by over-cooling the natural gas; a mixed refrigerant forming operation forming a mixed refrigerant by mixing the second expanded refrigerant and the first expanded refrigerant; and a cooling operation cooling the natural gas, the first separated refrigerant, and the second separated refrigerant which were introduced into the first heat exchanging region by supplying the mixed refrigerant to the first heat exchanging region, and forming the first separated refrigerant at high temperature by cooling less the first separated refrigerant than the natural gas and the second separated refrigerant.
 9. The method for liquefying natural gas of claim 8, wherein in the first pre-cooling operation, the first mixed refrigerant and the natural gas are pre-cooled using a single refrigerant or a second mixed refrigerant.
 10. The method for liquefying natural gas of claim 8, further comprising: a converting operation converting the mixed refrigerant into the first mixed refrigerant by sequentially compressing and cooling the mixed refrigerant; a second pre-cooling operation pre-cooling the first mixed refrigerant and the natural gas; and a repetition cycle operation repeating the first mixed refrigerant separating operation to the second pre-cooling operation as one period one or more times.
 11. The method for liquefying natural gas of claim 10, wherein in the second pre-cooling operation, the first mixed refrigerant and the natural gas are pre-cooled using a single refrigerant or a second mixed refrigerant. 